I was testing things out trying to get more details, and I think I may have found what was causing it.
If you're using FireFox and have the game full-screened, you can apparently click-and-drag from the left of the page to the right and select the full-screen window? I looked around for another game to test this with and found the same results with another Unity game or two. Seems to be an odd interaction between Unity and FireFox.
(Tested on Chrome as well, the bug is nonexistent. No blue at all.